In recent years, a version of this practice called intermittent fasting, where people skip eating anywhere from several hours to several days in a row, has started taking off. Hugh Jackman once said he only eats for a strict 8 hours each day, and Silicon Valley biohackers are embracing a 36-hour water-only "Monk fast," as they call it, which some perform once a week.

Another popular version of the plan, the 5:2 fast, lets people eat normally most of the week, but then requires a strict limit of around 500 calories per day on the remaining 2 days.

There's clear evidence that fasting, when done right, can reduce a person's chances of developing long-term health issues like diabetes, heart disease, and multiple sclerosis (MS). And it helps some people lose weight, too.

Dr. Miriam Merad, director of the Precision Immunology Institute at Mount Sinai School of Medicine, said the typical modern diet of constant eating is making our immune system cells work overtime, and it's not good for long term health. Her team's small study (in both people and mice) out in the journal Cell today provides some of the first essential clues about why letting our guts spend many hours of the day without food can do a body good.

Eating food turns on inflammatory cells in the body

An Indian Muslim devotee preparing Iftar meals to be distributed for breaking the fast at Nakhoda Masjid in Kolkata, India on June 4, 2019.
Indranil Aditya/NurPhoto via Getty Images

The reason why fasting is good for us has to do with a type of immune cell called a monocyte, which our bodies typically release to fight off infections and wounds.

Monocytes are inflammatory, and the white blood cells can cluster to heal the body when we're injured. But any time we eat food, monocytes are also standing guard in case we ingest any threatening microorganisms. This is especially true when we eat (and drink) sugar. Monocytes also accumulate in fat tissue, contributing to chronic disease.

Merad's new study provides some of the first evidence that intermittent fasting can help calm these inflammatory cells, making them less active. By taking blood samples from 12 healthy adults who were asked to fast for 19 hours in a day (and performing similar experiments with similar results on mice, too), Merad's team of scientists discovered that the subjects' circulating monocyte levels were astonishingly low while fasting.

"That scared us, because we thought maybe when you diet like this, if you have an infection, this monocyte won't be able to react to it," Merad said. This turned out not to be the case.

Her hunch is that by being well-fed every day, we are creating a perfect storm of inflammatory monocytes running on overdrive in the body, setting people up for chronic diseases like diabetes, heart disease, and liver issues, especially if we run on lots of sugar.

When we fast, though, we deprive our bodies of glycogen, a simple energy source that often comes from carbohydrates like sugar.

"Usually the number of monocytes that we have circulating is pretty high because, in America especially, we eat all the time, Merad said. "We snack. I don't know whether you snack, I snack all the time."

Don't start a fasting plan without expert advice

Merad cautions that her study should not serve as dietary advice on its own.

"All fasting has to be done in discussion with a dietitian, with a nutritionist, with your general practitioner," she said.

She said it's important to understand the difference between fasting and starving, which can cause long-term brain damage or even be deadly. People who are particularly sensitive to glucose levels (like diabetics) and other at-risk groups, including pregnant women, likely should not fast.

Fasting is not 100% safe for anyone when taken to extremes.

"If you start fasting for too long you destroy your immune system," Merad said. "You become very susceptible to infection. So fasting is not a trivial thing. It's good to fast, but you cannot starve yourself."

Merad now tries to eat dinner a few hours earlier than she used to, and she said other researchers in her lab (as well as her husband) are also experimenting with their own versions of intermittent fasting plans, like skipping breakfast.

"Often we eat because we want some social time with family and friends, but do we need to eat three times a day?" she asked. "Maybe eating two times a day would be entirely sufficient and very beneficial, in fact, in terms of health."